1. Field of the Invention
The present invention relates to optical receiver circuits which include a triggered inverter stage for use in circuits having a noisy supply voltage. More specifically, the present invention is directed to a triggered receiver circuit that can be used on a dense optoelectronic-VLSI circuit which combines analog optoelectronic I/O devices with high-density digital logic circuits on the same integrated circuit substrate.
2. Description of Related Art
Optical receivers have the task of first converting the analog intensity-modulated optical energy emerging from the end of a fiber or other optical signal carrier into a digital electrical signal, and then amplifying the electrical signal to suitable logic voltage levels for further processing on the integrated circuit chip. To accomplish their functions, optical receivers generally include a photodetector, an amplifier, and signal processing circuitry. Electrical currents generated by the photodetector of the receiver are typically very weak and are therefore adversely affected by random noise on the input signal as well as in the power supply. This is especially true when the optical receiver is mounted close to a digital CMOS circuit which is a significant source of digital noise and crosstalk.
The optical signal experiences many obstacles which may corrupt its original form, such as attenuation and distortion in the optical fiber, photodetector noise in the photodetector and supply noise such as voltage pulses and amplifier noise in the amplifier.
Transimpedance amplifiers, which have a transimpedance feedback element, were developed to provide optical receivers with good sensitivity and dynamic range. A typical input output relationship of a transimpedance amplifier is shown in FIG. 1a, with the vertical portion of the relationship showing the switching threshold. Transimpedance receivers, which include a transimpedance amplifier, are typically biased very close to the switching threshold of the transinpedance amplifier. In a differential two-beam operation, the receiver is actually biased at the midpoint of the switching threshold of the transimpedance amplifier, allowing the receiver to be sensitive to small changes in the input voltage and hence be sensitive to the small amounts of photocurrent (the weak signal) operatively generated by the light detector. The output of the transimpedance amplifier is transmitted to further gain stages which amplify the detector output to low and high logic output levels.
This conventional design is well suited for low-noise systems or for small arrays, but lacks an adequate tolerance to variations in the supply voltage, i.e. supply noise, which may be associated with ground and power lines of high density VLSI circuits due to the simultaneous switching noise, ground bounce and cross-talk from the digital part of the circuit. This noise may be transmitted to the input lines and power supply lines of the receiver and cause the detection of erroneous or spurious signals by the receiver. Since the supply noise is amplified with the weak electrical signal generated by the photodetector, relatively small amounts of input and supply noise can easily corrupt the signal. And inasmuch as the addition of circuitry and related current draws on an already crowded circuit would only add to the problem to be avoided, it would be advantageous to be able to control the deleterious effects of supply noise in the receiver using the least possible amount of additional circuitry and power consumption.